pgsql: Remove documentation mention that hash indexes perform no better

Log Message:
-----------
Remove documentation mention that hash indexes perform no better than
btree; keep mention about missing WAL logging.

Kenneth Marshall

Modified Files:
--------------
pgsql/doc/src/sgml:
indices.sgml (r1.75 -> r1.76)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/indices.sgml?r1=1.75&r2=1.76)